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I think you might want a much stronger mixer for blending ambiances as well. That'll chew up more HP than what you have... but might be worth it. Roland makes a six channel mixer with pans and mutes that's in my "must buy" list. It's a lot bigger. But being able to mix/pan up to six sound sources as well as mute them might be pretty handy for ambient mixes.
Hi, @Ronin1973. What's the model of that Roland mixer? Is it this SYSTEM-500 530 dual-VCA with six input sliders?